The present invention relates to a hermetic sealed container, more particularly to a container which is filled with a positive pressure generating contents and hermetically sealed with an easy-opening lid.
Conventionally, the easy-opening lid used for a hermetic sealed container filled with a contents, such as beer or carbonated soft drinks, that generates internal pressure higher than the normal atmospheric pressure in the sealed container is usually of the type that is provided with a rigid, substantially flat, metal panel portion formed with a score line for defining an opening, and a pull tab secured to the panel portion for tearing the score line to open the container.
On the other hand, another easy-opening lid of the type that is provided with a substantially flat panel portion formed with an opening, and a peelable seal tab bonded to the outer surface thereof to cover the opening is also conventionally used for the hermetic sealed container filled with a negative pressure generating contents, such as juices and soups.
The latter easy-opening lid of the seal tab type is more advantageous than the former of the score type in more simple manufacturing procedures and lower manufacturing cost. However, in the case of the hermetic sealed container filled with a positive pressure generating contents, there arise problems that, when the seal tab is bonded with a lower adhesive strength so as to facilitate peeling the seal tab in opening the container, the tab tends to be subjected to spontaneous peeling due to the internal pressure during storage, and when the seal tab is bonded with a higher adhesive strength so as to prevent the tab from peeling spontaneously, the seal tab is hard to peel in opening the container.
In addition, a lid formed of a flexible laminate comprising a metal foil, such as aluminum foil, and a thermoplastic film has been recently employed, since the lid has advantages that it can be bonded by heat sealing to a container body formed of a thermoplastic material or a laminate including the latter, and has gas barrier capability. When a negative pressure generating contents is filled, the lid of the last-mentioned type may be used as a score type, easy-opening lid formed with a score line which reaches the metal foil, or as a seal tab type, easy-opening lid.
When a positive pressure generating contents is filled, however, the lid is difficult for practical application in case of the score type, because the score line is apt to be ruptured by the relatively high positive internal pressure, and in case of the seal tab type, the above-described problems may be encountered.